The π back-donation influence in CO oxidation on small and oxidized Au–Ag clusters

Abstract

CO oxidation catalyzed by bimetallic Au–Ag clusters. • The π back-donation as an indicator of reactivity in CO oxidation reactions. • The π back-donation depletion in the M−CO interaction promotes CO oxidation. • The bimetallic Au 2 Ag 2 presents the lowest energy barrier for CO 2 formation. • AuAg represents an improvement over pure Au and Ag clusters in CO oxidation. Carbon monoxide is a pollutant that can be removed by oxidation in catalytic converters. This work presents CO oxidation on small and oxidized Au n , Ag m , and bimetallic Au n Ag m ( n + m ≤ 6) clusters using theoretical calculations by the ZORA-DFT approach. During CO adsorption, the Mayer bond order follows the trend: Au–CO > AuAg–CO > Ag–CO. The CO adsorption on bimetallic AuAg clusters decreases π back-donation, which reduces the M−CO bond covalent nature. The bimetallic II Au 2 Ag 2 presents the lowest energy barrier for the CO 2 formation, 0.08 eV, where the Langmuir-Hinshelwood mechanism constitutes a viable route for CO oxidation. Bimetallic Au–Ag represents an improvement compared to pure Au and Ag clusters because they reduce the M−CO bond’s covalent nature, causing weaker M−CO bonds. These factors reduce the activation energy of CO oxidation. We propose the π back-donation as an indicator of cluster reactivity.